1. Field of the Invention
The present invention concerns the technical area of composite products, more particularly, products used for the fabrication of molded articles or parts.
2. Description of Related Art
Composite molded articles are light and at the same time strong. They are used in a wide variety of applications, including: aeronautical construction, for example for the design of airplane wings; aerospace and naval construction, for example for the fabrication of ship masts or hulls; automobile manufacturing; the mechanical electrical industries; and the fabrication of sports articles such as skis, tennis rackets and sail boards.
One of the fabrication techniques for molded articles involves using prepregs formed of an impregnated fibrous reinforcement, generally a resin that associates a thermosetting polymer and a cross-linking agent, also called a hardening agent, and possibly additives that serve to improve the performance of the molded article obtained.
The prepregs are laid up in a mold so as to obtain the desired shape and linked to each other by cross-linking the thermosetting polymer.
However, the prepregs are difficult to store at room temperature. Indeed, the fact that fibrous reinforcement is impregnated with a homogenous mixture of thermosetting polymer and its cross-linking agent at room temperature leads to a cross-linking reaction, slow, to be sure, of the thermosetting resin.
Thus, a major inconvenience associated with the use of such prepregs is that they require cold storage to minimize the change in the resin during storage. In addition, cross-link reaction control processes are often necessary when prepregs are used in designing molded parts. The consequence of the change in the resin is that the prepregs must be used rather quickly or they become unusable. Solutions that provide composite products in which the resin matrix has good storage qualities at room temperature have therefore been sought.
U.S. Pat. No. 3,666,615 describes the combination of a layer of thermosetting resin and a layer of hardener in which the two layers are separated by a film that prevents any contact between the resin and the hardener. The film is made of a material that melts when heated. Thus, raising the temperature eliminates the physical barrier made by the film and establishes contact between the thermosetting resin layer and the hardener layer which can then react. A fibrous base is included in the thermosetting resin layer or in the hardener layer and fills the function of a strengthener. Nevertheless, this technique requires using a film of a different type from the thermosetting resin, which complicates the system.
The application for a European patent published under number EP 1 072 634 uses adjacent layers of reactive materials, but in this case, there is no physical barrier separating the resin and the cross-linking agent. Therefore, the cross-linking reaction may take place at the interface of the layers during storage.
It appears then that the preceding techniques are not entirely satisfactory. In that context, there is a need for composite products comprising first, a reinforcement structure and second, a system combining a thermosetting resin that can undergo a cross-linking reaction. This product should provide satisfactory stability at room temperature, thus facilitating storage.